Switching devices in general are well known to the person skilled in the art, in particular concerning their architecture, their implementation and specifically their implementation for switching digital signals.
A general structure for switching networks can be found in an article by C. Clos entitled "A study of non-blocking switching networks" in BSTJ Vol. 32, No. 2, March 1953, pages 406-425. An article entitled "Switching, synchronizing and signalling in PCM exchanges" by W. Neu and A. Kuendig at the Colloque International de Commutation Electronique held in Paris, Mar. 28th to Apr. 2nd 1966 at pages 513-520 as published by Chiron in Paris, relates more particularly to a time-division switching system.
These low data rate devices implement relatively slow technologies and do not take account of the constraints specific to processing fast digital signals, which constraints are:
implementing high performance technology;
improving inter-stage links, since processing high speed signals require short transit times, in particular in transmission systems between stages or between cards;
signal regeneration, since in spite of the precautions taken in suitably choosing the technology for transmission systems, the shape of the digital signals is progressively degraded and it is essential to regenerate these signals both in amplitude and in phase; and
the supervision system: if a passive monitoring system is chosen for reasons of economy and simplicity of implementation, the device must take into account the fact that the time taken to transfer data from one point to another of the switching network is large and indeterminate relative to the period of the signals being transferred.
The supervision of low data rate devices is generally based on a communication monitoring process which verifies, either continuously or by sampling:
that the paths taken match the call data; and
that the various transmission means and switching means implicated in a particular path are functioning correctly.
Continuous monitoring of communications can only be performed in duplicated systems operating in microsynchronism, i.e. phase synchronized at the bit level.
Monitoring by sampling avoids the need to duplicate the switching network and can be done in two different ways:
active monitoring consists in inserting known data to the input of the network and in observing the data which arrives at the output, and optionally at various intermediate points; and
passive monitoring consists in observing various points along the itinerary of a normally connected signal or a representative sample of said signal and in comparing these signals or samples as obtained from the various points.
In broad band networks, continuous monitoring is difficult to implement because of technical constraints associated with operating in microsynchronism.
In active monitoring, injecting a code in a reserved sector and recognizing it in analysis systems requires the entire signal to be processed and thus leads to apparatus which is particularly complex and expensive.
In passive monitoring, a variable number of signal selectors need to be installed in the switching chain, together with comparators. In practice, it is impossible to directly compare high data rate signals because of the technological complexity required to transmit such signals and to compare them.
Further, compared with conventional methods of supervising switching networks, a new parameter must be taken into account: this parameter is the error rate of the monitored connection. Taking account of the component parts of a broadband connection chain, it is important to be able to evaluate the quality of a connection which is considered as being properly established in order to be able to monitor any changes or drift in said quality.
Preferred embodiments of the invention provide a switching network suitable for high data rate digital signals, together with a device for monitoring the high data rate digital signals.